The flexible motion segments of the human spine include at least a pair of adjacent vertebral bodies, an intervertebral disc adhered therebetween to provide a cushion and bind the vertebral bodies together, and a pair of facet joints, which are small stabilizing joints located between and behind the adjacent vertebral bodies. The facet joints and the intervertebral disc, with the help of various ligamentation, function in harmony as a three joint entity to prevent excessive motion, over-twisting, or toppling over of the spine and to preserve the flexibility needed to turn, look around, and move about.
Millions of people suffer from back pain. In some instances, in order to relieve back pain and/or to stabilize the spine, it becomes necessary to insert an intervertebral implant into the intervertebral disc space between adjacent vertebral bodies at one or more levels. Current implants however do not take into account all of the biomechanical aspects of the human spine, especially the interplay between the intervertebral disc and the facet joints. Generally speaking, intervertebral implants modify the patient's normal physiological spinal motions with consequences. For example on the actual range-of-motion of the segment or on shear loads acting on the different spine components (e.g., facet joint, facet capsule, ligaments, bone-implant interface, etc.). Thus current intervertebral implants do not result in a completely satisfactory solution. That is, current implants, whether unconstrained, semi-constrained or constrained, do not mimic an important feature of the intervertebral disc, namely, the visco-elastic dampening ability. This may lead to a redistribution of mechanical loads between the intervertebral disc and the facet joints. Moreover the insertion of rotation centers without lateral translations modifies the dynamics of the spine.
Thus, it is desirable to design and manufacture an intervertebral implant that better mimics the physiological motions of the natural spine.